Ultrasonic techniques for the mechanical characterization of viscoelastic materials such as soft tissue have grown in interest due to their clinical relevance to monitoring the progression of various diseases [1]. These techniques include ultrasound elastography monitoring of strain in response to both extrinsic and intrinsic forces. Several researchers have developed elastography methods to render images of local strains by applying a relatively uniform, external compression to tissue and tracking subsequent tissue displacements [2]-[16]. The elastic modulus of tissue can be estimated using these methods with minimal complexity, providing an intrinsic measurement of the tissue's material properties. However, because direct compression of tissue is required, elastography can be challenging when attempting to access tissue superficial to boundaries such as organ or vascular layers. Elastographic methods that monitor tissue response to intrinsic forces, such as cardiac pulsation have also been developed but with small strains associated with poor contrast in parametric images [17]-[19].
One possible alternative method that has been explored involves tracking local strains in tissue through acoustic radiation force imaging. Rather than relying on external compression, acoustic radiation force methods use high-intensity ultrasound pulses to transfer momentum to tissue [20]-[31]. By direct application of focused radiation force at the point of interest, these methods allow for measurement of tissue responses superficial to boundary layers. Several techniques involve monitoring the dynamic response of tissue to impulsive radiation force. In acoustic radiation force impulse (ARFI) imaging, tissue displacements are generally tracked axially after the transmission of a temporally short (e.g., <1 ms), focused acoustic radiation force excitation. Resulting tissue displacement data are typically illustrated through a set of parametric images that include peak displacement and time to recovery. Although these parameters have been shown to be inversely related to the Young's modulus in homogeneous elastic media [30], only the relative stiffness or compliance of tissue can be assessed from ARFI imaging because the magnitude of radiation force is generally unknown. That being said, the generation of impulsive tissue excitation also results in the initiation of shear wave propagation traveling perpendicular to the applied force. Shear wave elasticity imaging (SWEI) produces force-independent images of the reconstructed shear moduli of tissue by monitoring shear wave speed [28], [32].
Other applications of acoustic radiation force imaging include monitoring of the resonant response to excitation as in vibro-acoustography [20] or harmonic motion imaging [24]. In kinetic acoustic vitroretinal examination (KAVE) as developed by Walker et al. [22], [23], multiple acoustic pulses per lateral location are generated with a single element piston transducer to observe the steady-state response of tissue to acoustic radiation force. Assuming that the forcing function is a temporal step function and tissue can be described discretely as a Voigt model, images can be generated of force-free parameters depicting the time constant, damping ratio, and natural frequency of the examined homogeneous tissue mimicking material.
Although these techniques can be useful in identifying certain characteristics of the tissue, most of the results obtained are qualitative rather than quantitative. For example, using existing ultrasound measurement techniques, elasticity and viscosity of a sample can be determined relative to that of other samples, and not in absolute numbers. Accordingly, in light of these difficulties with associated with conventional acoustic radiation force measurement techniques, there exists a need for improved methods, systems, and computer readable media for monitored application of mechanical force to samples using acoustic energy and mechanical parameter value extraction using mechanical response models.